Blender python custom UI button run custom function?
You can use custom properties in Blender by defining it like so (for example):

bpy.types.Scene.your_property = bpy.props.IntProperty(
    name = "Name your int prop", # as you want it to appear on the slider
    default = 1, # optional
    description = "Short description of your prop, to show in the popup"
    )


or

bpy.types.Scene.your_property = bpy.props.StringProperty(
    name = "Name your prop",
    default = "default value for your prop (optional)",
    description = "Short description of your prop, to show in the popup"
    )


You can read this for documentation about properties: http://www.blender.org/documentation/blender_python_api_2_68_2/bpy.props.html

Later, you can use your property in the code, by calling it like so (for example, and assuming you're in a function with context available, like your invoke function of your MakeTarget class) :

if context.scene.your_property == 2:
    do_something()


And, to use it in Blender's UI (TargetMakerPanel class):

row = layout.row()
row.prop(context.scene, "your_property")


Blender will automatically make the UI that correspond the property type. (eg. a slider for an IntProperty, input box for a string, etc.)

You can see more examples here : http://wiki.blender.org/index.php/Dev:2.5/Py/Scripts/Cookbook/Code_snippets/Interface#Using_scene_properties_to_store_information

Hope this helps!